An imaging device, or sensor, is a light-sensitive electronic component that converts electromagnetic radiation into an analog electrical signal. This signal is then amplified and digitized by an analog-to-digital converter and finally processed to obtain a digital image.
The imaging device uses the photoelectric effect, whereby incident photons tear electrons from each active element, called a photosite. An imaging device generally comprises photosites arranged in a matrix, each photosite corresponding to a pixel of an image. The photons, sensed by the semiconductor-component-based imaging device, are converted into electron/hole pairs in the silicon. More precisely, the charge created in the light-sensitive regions is stored in the photosite before being output using an electronic system.
There exists two major families of imaging devices or sensors: charge-transfer sensors, otherwise known as charge-coupled devices (CCDs) and CMOS sensors, or active pixel sensor (APS) CMOS sensors. CCD sensors are the easiest to fabricate. They have good sensitivity, but the charge transfer is relatively slow due to their operating principle.
CMOS sensors comprise an integrated circuit incorporating cells containing light-sensitive regions, such as photodiodes, amplification, and shutter logic. This is in contrast to CCD sensors, which do not possess internal amplification and shutter electronics. They are more complicated to fabricate, but are produced using typical microelectronics techniques and therefore can be of substantial size. These CMOS sensors are widely used as autofocus sensors for digital reflex cameras.
A CMOS sensor provides an approach to the size and speed problems of passive image sensors, such as CCD sensors. CMOS sensors consume less power relative to CCD sensors, and also cost less to produce. In addition, owing to their structure, they combine image sensor and image processing functions.
In numerous image-capture-related applications, the subject of an image is likely to be moving at a constant velocity relative to the camera, and more particularly, relative to the sensor (imaging device). For example, the image of a moving object may be captured by a stationary camera, or the image of a stationary object may be captured by a camera in a moving vehicle, for example.
Consequently, in the case of an object having a high relative velocity with respect to a sensor, the integration time available to a photosite is correspondingly shorter. This is because, if the integration time of the photosites is too long, the same photons received by one photosite may also be received by the neighboring photosite, which may have collected other photons just beforehand. The photons then cover several photosites instead of only one, and the image obtained may be blurred.
A technique called time delay and integration (TDI) is used in line-scan image sensors when only a small amount of light is received, implying a long exposure time, or when the subject of an image has a high velocity. This technique makes it possible to integrate, over a longer time than the time available for integration in a single photosite, enough light when the subject of the image is moving relative to the sensor.
The TDI function has naturally been applied to CCD imaging devices, or sensors, by transferring a charge packet along the sensor synchronously with the movement of the image. This is because the intrinsic operation of a CCD sensor uses charge transfer. The transfer of the charge packet intrinsic to the operation of the CCD is synchronized with the relative movement of the subject of the image relative to the sensor so that the integration is carried out on the following photosite for the same light ray.
CMOS imaging devices do not use charge transfer from one photosite to another, as is the case for CCD sensors. The TDI function using charge summing cannot be carried out in typical CMOS sensors. U.S. Pat. No. 6,906,746 to Hijishiri et al. discloses the application of a TDI technique to a typical CMOS sensor. However, charge transfer being difficult in typical CMOS sensors, the TDI function does not result in an accumulation of charge, but in a summing of the voltages output by the photosites, corresponding to the charge respectively collected in each photosite. This voltage summing may lead to the summing of noise, interfering with the voltage measurements and degrading the image obtained.